The two-cell stage is a critical period of mouse early development when most of the maternal mRNA store is degraded and is replaced by a new mRNA population derived from the embryonic genome. Recent studies suggest that this transition results in a major qualitative shift in the mRNA population, with about one- half of the mRNAs being derived from maternal type genes (i.e., genes whose transcripts are stored in the egg) and the other half from newly activated zygote-specific genes. With an eventual aim to shed light on the mechanisms of transcriptional regulation, studies are proposed to characterize the transcriptional products and developmental pattern of expression of two groups of genes in the early embryo. (1) Using cDNA clones from a two-cell cDNA library as hybridization probes, a group of transcripts which are moderately abundant during the cleavage stages but appear to be absent or infrequent in the egg will be characterized with respect to transcript prevalence during development from oocyte to blastocyst, transcript size, and number of the corresponding genes in nuclear DNA. These studies are expected to identify a set of genes that show a clear zygote-specific expression (i.e., no detectable representation in oocyte and egg RNA) and thus may be suitable candidates for studies of transcriptional regulation. (2) Using isoform-specific probes, the developmental pattern of expression of the genes for cytoskeletal (beta and gamma) and muscle-specific (alpha-skeletal and alpha-cardiac) actins will be determined during the period from the late stages of oocyte growth through the blastocyst stage. The steady-state amounts of the various actin mRNAs will be assayed and their spatial distribution in early embryos determined. The results of this study should provide information on the developmental regulation of the actin gene family during early mouse embryogenesis.